Magnetic (or magneto-resistive) random access memory (MRAM) is a non-volatile access memory technology that could potentially replace dynamic random access memory (DRAM) as the standard memory for computing devices. Particularly, the use of MRAM-devices as a non-volatile RAM will eventually allow for “instant on” systems that come to life as soon as the computer system is turned on, thus saving the amount of time needed for a conventional computer to transfer boot data from a hard disk drive to volatile DRAM during system power-up.
A magnetic memory element (also referred to as a tunneling magneto-resistive or TMR-device) includes a structure having ferromagnetic layers separated by a non-magnetic layer (barrier) and arranged into a magnetic tunnel junction (MTJ). Digital information is stored and represented in the magnetic memory element as directions of magnetization vectors in the ferromagnetic layers. More specifically, the magnetic moment of one ferromagnetic layer is magnetically fixed or pinned (also referred to as a “reference layer”), while the magnetic moment of the other ferromagnetic layer (also referred to as “free layer”) is free to be switched between the same and opposite directions with respect to the fixed magnetization direction of the reference layer. The orientations of the magnetic moment of the free layer are also known as “parallel” and “anti-parallel” states, respectively, wherein a parallel state refers to the same magnetic alignment of the free and reference layers, while an anti-parallel state refers to opposing magnetic alignments therebetween.
Depending upon the magnetic states of the free layer (i.e. parallel or anti-parallel states), the magnetic memory element exhibits two different resistance values in response to a voltage applied across the magnetic tunnel junction barrier. The particular resistance of the TMR-device thus reflects the magnetization state of the free layer, wherein resistance is “low” when the magnetization is parallel, and “high” when the magnetization is anti-parallel. Accordingly, a detection of changes in resistance allows a MRAM-device to provide information stored in the magnetic memory element, that is to say to read information from the magnetic memory element. In addition, a magnetic memory element is written to through the application of a bi-directional current in a particular direction, in order to magnetically align the free layer in a parallel or anti-parallel state.
An MRAM-device integrates a plurality of magnetic memory elements and other circuits, such as a control circuit for magnetic memory elements, comparators for detecting states in a magnetic memory element, input/output circuits and miscellaneous support circuitry. As such, there are certain microfabrication processing difficulties to be overcome before high capacity/density MRAM-devices become commercially available. For example, in order to reduce the power consumption of the MRAM-device and provide a variety of support functions, CMOS-technology is used. Various CMOS processing steps are carried out at relatively high temperatures, while ferromagnetic materials employed in the fabrication of MRAM-devices require substantially lower process temperatures. Thus, the magnetic memory elements are designed to be integrated into the back end wiring structure of back-end-of-line (BEOL) CMOS processing following front-end-of-line (FEOL) CMOS processing.
To be useful in electronic devices, very high density arrays, of magnetic memory cells are utilized in magnetic random access memories. In these high density arrays, the magnetic cells are generally arranged in rows and columns. Individual cells are addressable for reading and writing operations by the selection of an appropriate row and column containing the desired cell. Also, conveniently orthogonal current lines are provided, one for each row and one for each column, so that a selected cell is written by applying current to the appropriate row current line and the appropriate column current line.
Recently, and especially in view of modem portable equipment, such as portable computers, digital still cameras and the like, the demand of low-cost and high-density mass storage memories has increased dramatically. Therefore, one of the most important issues for low-cost and high-density NRAM-devices is a reduction of the MRAM-cell size. However, down-scaling MRAM-cells requires smaller and smaller magnetic tunnel junctions and, therefore, a lot of problems can arise. For a given aspect ratio and free layer thickness, the activation energy is dependent on the free layer volume scales down, like w, where w is the width of the magnetic cell. Otherwise, the switching fields increase roughly, like 1/w. Thus, in scaling down MRAM-cells field selected switching becomes ever harder, but at the same time the magnetic cell looses its information more and more rapidly due to thermal activation. A major problem with having a small activation energy (energy barrier) is that it becomes extremely difficult to selectively switch one MRAM-cell in an array. Selectability allows switching without inadvertently switching other MRAM-cells.
In general, if a magnetic field in the direction opposite to the magnetization direction of the free layer is applied in the direction of the easy axis of the magnetization, then the magnetization direction is reversed to the direction of the applied magnetic field at a critical magnetic field value, which is also referred to as reversal magnetic field. The value of the reversal magnetic field can be determined from a minimum energy condition. If a magnetic field is applied not only in the direction of the easy axis of magnetization but also in the direction of the hard axis of magnetization, then the absolute value of the reversal magnetic field decreases. In particular, where the magnetic field applied to the direction of the hard axis of magnetization is represented by Hx and the magnetic field applied to the direction of the easy axis of magnetization is represented by Hy, then a relationship Hx(2/3)+Hy(2/3)=Hc(2/3) is established, where Hc represents the anisotropic magnetic field of the free layer. Since this curve forms an asteroid on the Hx-Hy-plane, it is called an asteroid curve. As can be seen from the above relationship, a composite magnetic field enables the selection of a single NRAM-cell positioned at an intersection of word and bit lines in the position where only the sum of both magnetic fields at least amounts to the reversal magnetic field.
A typical switching mechanism used for switching MRAM-cells is the well-known “Stoner-Wohlfahrt”-switching scenario, in which magnetic anisotropy of the free layer is chosen to be approximately parallel to a wafer surface. In particular, writing into an MRAM-cell is performed by controlling the magnetization direction of the free layer using a composite magnetic field generated by supplying current to both of a word line and a bit line. Another method of switching an MRAM-cell is the well-known “adiabatic rotational switching”-scenario, in which magnetic anisotropy of the free layer is chosen to be inclined under an angle of about 45° relative to the wafer surface. The rotational switching mechanism is, for example, disclosed in U.S. Pat. No. 6,545,906 B1 to Savtchenko et al., the disclosure of which is incorporated herein by reference. One key difference between Stoner-Wohlfarth-switching and adiabatic rotational switching is that the latter one uses only uni-directional fields.
In light of the above, it is an object of the invention to provide a magnetic memory element and magnetic random access memory (MRAM) device comprising such magnetic memory elements allowing a cell-size down-scale without thereby causing severe problems as to an increase of switching-fields and decrease of activation energy.